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To meet the requirements, automakers would need to add speakers that are audible from the street.

Adding speakers

Automakers would need to make hybrids and electric vehicles emit sound under rules that the National Highway Traffic Safety Administration proposed on Monday

The rules, ordered by Congress three years ago, are meant to protect pedestrians and bicyclists from vehicles that make little sound when using electric power. NHTSA says that designing the vehicles to make noise at speeds below 18 mph would prevent about 2,800 injuries over the life of each model year of vehicles.

Adding the needed speaker system would increase the cost of manufacturing a car or light truck by about $30, the agency estimates. NHTSA estimates it would cost the whole industry $23 million in 2016, once the rules are in effect.

"Our proposal would allow manufacturers the flexibility to design different sounds for different makes and models while still providing an opportunity for pedestrians, bicyclists and the visually impaired to detect and recognize a vehicle," NHTSA Administrator David Strickland said in a statement.

Before finalizing the rules, NHTSA must publish them in the Federal Register and address any problems that are brought up during a subsequent 60-day public comment period.

The rules would apply to passenger cars and light trucks, as well as motorcycles, heavy-duty trucks and buses.

Compelling argument

To meet the requirements, automakers would need to add speakers that are audible from the street, but still protected from the elements. These speakers would need a digital processor so they would play the chosen sound -- often a humming noise similar to that of a gasoline-burning engine -- only at low speeds.

Beyond the cost of those components, the added weight would increase fuel costs by about $5 over the lifetime of a light vehicle, NHTSA says. That, combined with the $30 in components, means the total cost of a vehicle would increase by about $35.

Automakers have started adding speakers to hybrids and electric vehicles. The 2013 version of the Chevrolet Volt, the best-selling plug-in hybrid on the market, lets the driver activate a warning sound using a button on the end of the turn signal lever.

The Nissan Leaf, the best selling battery-electric vehicle, has a similar system that plays a sound at speeds of up to 18 mph. It plays automatically, but a driver can deactivate the sound by pressing a button beneath the navigation screen.

And starting with the 2012 model year, all U.S. versions of the Toyota Prius, the best-selling gasoline-electric hybrid, automatically make an electronic whirring sound. The sound plays at speeds below 15 mph.

Toyota does not let drivers disable the sound on its hybrids, as advocates for the blind and the elderly have insisted is necessary.

Regulators said on Monday that they find that argument compelling. During a visit to the headquarters of the National Federation of the Blind, NHTSA officials tried to cross city streets while blindfolded. They "found the sound of idling vehicles necessary for determining whether there was a vehicle present at the intersection and whether it was safe to cross," the proposal says.

Under the proposal, drivers would not be able to deactivate a warning sound while a vehicle is in motion, which NHTSA says "would compromise pedestrian safety."

But the proposal says regulators have not yet decided whether vehicles should be required to make noise while idling. Though some drivers have chafed at hearing noise instead of near-silence, automakers have largely recognized the risk quiet cars can present to pedestrians.

Yet the rules will require some automakers to add more equipment, and others to change how they design their warning systems.

Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, said the group had no immediate comment because it is still reviewing the proposal.

She wrote in an e-mail on Monday: "We have been working closely with the blind community and NHTSA on this issue for several years and are continuing to do so to achieve a balanced and effective rule."